Marine microbes are central to ocean ecology and global
biogeochemistry. For instance, oxygenic photosynthesis, which is dominated by
microalgae in the oceans, converts gaseous CO2 into organic compounds,
whereas respiration, largely dominated by heterotrophic bacteria,
accomplishes the reverse. The fraction of organic carbon that escapes
bacterial re-mineralization contributes to reduce Earth’s lithosphere,
a process that, over geological time scales, has raised the amount of
free-oxygen in the atmosphere to levels that enable present-day life.
Calcification by marine microbial calcifiers such as coccolithophores and
foraminifera converts dissolved inorganic molecules to solid-phase calcite
and aragonite, a process that contributes to modulate atmospheric CO2 levels,
ocean alkalinity and therefore Earth’s climate. Moreover, marine
microbes sustain global fisheries as the base of marine food webs, contribute
to keep marine ecosystems in good health and provide an array of natural
products widely used in pharmaceutics and other industries with enormous
socio-economic impact. Life on Earth is microbe dependent, yet little is
known about the mechanisms that control the assembly of microbial communities
and their diversity.

In our recently finished project
TASIO, we have explored the global distribution of two key phytoplankton functional
groups diatoms and coccolithophores. In Tasio we studied how phytoplankton
such as diatoms and coccolithophores play a part in the regulation of
atmospheric carbon dioxide, which has significance to important climate
change studies.

Carbon uptake by
phytoplankton, and its export as organic matter to the ocean interior (a
mechanism known as the ‘biological pump’) lowers the partial
pressure of CO2 in the upper ocean and facilitates the diffusive
drawdown of atmospheric CO2. However, precipitation of calcium
carbonate by marine calcifiers such as coccolithophorids increases the
partial pressure of CO2 and promotes outgassing from the ocean
to the atmosphere (known as the ‘alkalinity pump’). Over the
past 100 million years, these two carbon fluxes have been modulated by the
abundance of diatoms and coccolithophorids, resulting in biological
feedback on atmospheric CO2 and Earth’s climate.(PNAS 2008).

Dispersal is a life-history trait that has profound consequences for
individuals, populations and communities, and its importance has been well
recognized for centuries. Viewed from an evolutionary perspective,
dispersal determines the level of gene flow (as opposed to genetic
isolation) between individuals and populations, and affects processes such
as local adaptation, speciation, extinction, and the evolution of
life-history traits. We have shown recently that marine diatom species
possess global dispersal ranges. Our observations revealed that diatom
communities from North Atlantic resembled much more to those of the North
Pacific than to those in the Southern Ocean despite being separated by
continental masses (Science 2009).

Ubiquitous dispersal
facilitates environmental tracking and habitat recolonization. During the
Pleistocene there were massive changes in climate that altered the
distribution and areal extent of oceanic biomes. Analyses of fossil records
showed that the taxonomic composition of marine diatom communities may
change and recover in concert with the glacial/interglacial climates of the
Pleistocene, demonstrating that disruption of local conditions leads
microbial species to disperse into favourable habitats elsewhere. This
mechanism allowed these marine microbes to survive across dramatic climate
events in the geological past. See PLoSone 2010

• PROJECT TITLE: Phytoplankton abundance
and cell size in the ocean

Phytoplankton abundance and cell size

Large sized species
with a higher surface to volume ratio are at a disadvantage with respect to
smaller cells for nutrient uptake in nutrient-poor ecosystems. Contrary to
expectations, the relationship between population density and cell size
exhibited a power function with an exponent near -0.75 regardless of the
nutritional status of the system. These results are consistent with a
Darwinian evolutionary model, in which larger species evolve adaptive
strategies to cope with their biophysical limits to nutrient acquisition. (Eco Letts
2006)